package com.wc.AlgoOJ_homework.AlgoOJ_BL1.E_翻转;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

/**
 * @Author congge
 * @Date 2024/3/30 12:24
 * @description http://43.138.190.70:8888/p/1092?tid=65f926e1f748afd09501f854
 */
public class Main {
    static FastReader sc = new FastReader();
    static PrintWriter out = new PrintWriter(System.out);
    static int N = 1000010;
    static char[] s;
    static char[] t;
    static int T;

    public static void main(String[] args) {
        T = sc.nextInt();
        while (T-- > 0) {
            t = (" " + sc.next()).toCharArray();
            s = (" " + sc.next()).toCharArray();
            int n = s.length - 1;
            if (s[1] != t[1] || s[n] != t[n]) {
                out.println(-1);
                continue;
            }
            int res = 0;
            boolean flg = false;
            for (int i = 2; i < n; i++) {
                if (s[i] != t[i]) {
                    if (s[i - 1] != s[i] && s[i + 1] != s[i]) {
                        res++;
                        s[i] = t[i];
                    } else {
                        flg = true;
                        break;
                    }
                }
            }
            if (flg) out.println(-1);
            else out.println(res);
        }
        out.flush();
    }
}

class FastReader {
    StringTokenizer st;
    BufferedReader br;

    FastReader() {
        br = new BufferedReader(new InputStreamReader(System.in));
    }

    String next() {
        while (st == null || !st.hasMoreElements()) {
            try {
                st = new StringTokenizer(br.readLine());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return st.nextToken();
    }

    int nextInt() {
        return Integer.parseInt(next());
    }

    String nextLine() {
        String s = "";
        try {
            s = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return s;
    }

    long nextLong() {
        return Long.parseLong(next());
    }

    double nextDouble() {
        return Double.parseDouble(next());
    }

    // 是否由下一个
    boolean hasNext() {
        while (st == null || !st.hasMoreTokens()) {
            try {
                String line = br.readLine();
                if (line == null)
                    return false;
                st = new StringTokenizer(line);
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return true;
    }
}
